Well technically, the slipping away of Sora's memories was merely due to him loosing himself to the darkness. Roxas doesn't really start getting any of Sora's memories until about a week later (Day 7) when Sora enters Castle Oblivion.
Usually when a Nobody and Heartless are created, the Nobody retains the will and body, whereas the Heartless retains the heart. Since there was more than one heart within Sora, this process got confused: the Heartless retained the will in addition to the heart; the Nobody (Roxas) got a body and heart, although neither of those were necessarily Sora's. (That's the simple version.)
In the case of the Seeker of Darkness, he didn't actually have the full-blown human form until he possessed Riku—before that he looked much more like a normal Heartless (that slumped over hooded figure). And while he didn't necessarily retain the apprentice's memories either, according to Dream Drop Distance, [Xehanort's] objective is etched into his heart, thus his Heartless and Nobody merely have the same goals.
I hope that made sense—I'm not really good at explaining things x_x